Insulin-like growth factors (IGFs) have long been familiar as somatomedins, synthesized in and released from the liver in response to growth hormone stimulation, and acting in an endocrine fashion to promote the growth of peripheral somatic tissue. Recent evidence suggests that IGFs may also have autocrine/paracrine roles in specific aspects of the growth and differentiation of the embryo and in the postnatal response of certain organs to injury. We have shown that during early embryonic development in the rat, IGF-1 mRNA is concentrated in target zones of the developing trigeminal and sympathetic nervous systems. IGF receptor mRNA has been localized to the ventral floorplate of the developing hindbrain and spinal cord, a region involved in axonal targeting and establishing the polarity of neural growth. Perinatally, IGF-1 mRNA was localized in maturing neurons of differentiating somatosensory systems. The topographic and temporal pattern of IGF-1 gene expression in these different settings is strongly suggestive of a role for IGF-1 in the development of neural connectivity. Further studies have shown that IGF-1 and IGF receptor mRNAs are localized in anatomically complementary regions of the adult rat kidney, adrenal and gonad, suggesting a possible role for IGF-1 in physiologic autoregulation of the size and function of these organs in postnatal life. Future directions include the investigation of the role of IGFs in neural development and plasticity and in mature organ hypertrophy with a view toward developing therapeutic applications for IGFs.